This ARPA-e SBIR project is to research, design, construct and field trial a cost-effective and efficient 25kW 200kWh energy storage system based on the all-Iron Hybrid Flow Battery (IFB) technology. Energy Storage Systems Inc. (ESS) develops and commercializes grid-scale energy storage systems based on flow battery technologies. Applying its proprietary flow cell design, ESS demonstrated a 4-fold performance improvement of the IFB technology at a 50cm2 lab-scale, and therefore the proposed ESS IFB stack is significantly smaller and cheaper than the conventional IFB stack. With support from Oregon BEST, Portland State Electric Avenue and Drive Oregon, ESS requests support from ARPA-e to scale up this lab-scale performance to a 25kW 200kWh system, and install and demonstrate the system behind the meter at a customer site. The customer can immediately expect, with no incentives, a payback of 5 years with a fully amortized cost of less than $200/kWh over 20 years, based on minimized time-of-use (TOU) and demand charges. Moreover, ESS has a financial model to project a development path to achieve less than $100/kWh per system. Besides the cost, this system can also provide the customer additional benefits, such as back-up power capability and provisions for demand response.